1. Field of the Invention
The present invention relates generally to the composition of an acoustic element of a speaker and more particularly to a speaker edge which is capable of lowering the lowest resonance frequency, f.sub.o of the speaker and of maintaining relatively stabilized sound characteristics of the speaker during variations of temperature and humidity.
2. Description of the Prior Art
In conventional cone type speakers, the speaker edges have been made of such materials as cotton fabrics, rubber sponges and foamed urethane resins. When cotton fabrics are used, the speaker edges have to be coated with an agent of high viscosity, generally known as a "vis-colloid". As a result, the characteristics of the speaker edges are greatly influenced by variations of temperature. When rubber sponges are used, it is difficult to produce edges having uniform characteristics because they are foamed from sheet materials. Finally, when speaker edges are made of foamed urethane, they are significantly affected by variations in humidity.
In order to overcome the above defects, extensive studies and experiments have been performed on new speaker edges and cone edges -- elements of a speaker which have a significant and delicate influence upon the overall sound characteristics. These efforts have centered on the use of expanded plastics obtained from ethylene copolymers, such as ethylene/vinyl acetate copolymer (EVA) and ethylene/ethyl acrylate copolymer (EEA), for example, because these materials are superior in strength and exhibit excellent flexibility at low temperatures and low brittle-point temperatures without the use of any platicizers. According to the results from these experiments, the characteristics of speaker edges formed from foamed EVA are somewhat improved over those of the conventional cotton cloth speaker edges at lower temperatures. However, the cotton cloth speaker edge is superior as a whole in other respects. It is expected that the internal loss of sound within the foamed plastic is excessive because of the use of the foamed plastic alone.
Consequently, it would be most desirable to have a speaker edge which is insensitive to changes in temperature or humidity while at the same time is capable of producing sound with high quality, uniform characteristics.